Version 6.1.0 is available upstream. Please consider updating! It seems that version 6.1.0 is available upstream, while the latest version in the Gentoo tree is 6.0.0. You think this warning is false? Read more about it here.
RtMidi is a set of C++ classes (RtMidiIn, RtMidiOut and API-specific classes)
that provides a common API (Application Programming Interface) for realtime MIDI
input/output across Linux (ALSA, JACK), Macintosh OS X (CoreMIDI, JACK),
and Windows (Multimedia Library) operating systems.
RtMidi significantly simplifies the process of interacting with computer MIDI hardware and software.